#!/bin/bash s=':account_id:12345:6789:Melbourne:Aus :account_id:98765:43210:Adelaide:Aus :customer_id:98765:43210:Adelaide:Aus' perl -pe 's~^(?:[^:]*:){2}(*SKIP)(?!)|:~~g if /^:account_id:/' <<< "$s"
Standard input is empty
:account_id:123456789MelbourneAus :account_id:9876543210AdelaideAus :customer_id:98765:43210:Adelaide:Aus